It's a twice in a lifetime moment: the transit of Venus across the Sun

On 6 June, an event that takes place only four times every two centuries will enthral the world's astronomers, as it has ever since the 1600s – but now it can provide priceless data in the hunt for habitable planets in deep space

The tiny black disc of Venus edges across the Sun during the last transit, in 2004. Photograph: Murdo MacLeod for the Observer

It's a twice in a lifetime moment: the transit of Venus across the Sun

On 6 June, an event that takes place only four times every two centuries will enthral the world's astronomers, as it has ever since the 1600s – but now it can provide priceless data in the hunt for habitable planets in deep space

A tiny speck will appear on one side of the Sun in a few weeks and slowly traverse the solar disc for a few hours. The movement of that little black dot may seem insignificant. But it is one of the rarest sights in astronomy, an event known as a transit of Venus. Miss this one and you will have to wait until 2117 for the next.

Earth's closest planetary neighbour, which is currently in close and spectacular alignment with Jupiter in the night sky, will make its passage across the Sun's disc on 6 June and can expect to make scientific headlines – for astronomers hope studies of the transit will provide them with key data for studying worlds that orbit distant stars.

"This transit is special because it is the last time in our lifetimes that we will have an opportunity to collect data for a planet as well characterised as Venus," said David Crisp of Nasa's Jet Propulsion Laboratory in Pasadena, California. "We will have to make the most of it."

Venus, for all its glittering beauty in the night sky and its association with the Roman goddess of love, is a deeply unpleasant world. It has a surface temperature of 460C, its dense atmosphere of carbon dioxide has incinerated or crushed all robot spacecraft that have landed on it and its surface is shrouded by thick clouds of sulphuric acid. Once thought to be a sister world to Earth, because of their similar sizes and orbits round the Sun, Venus is more like a vision of hell.

Nevertheless studies of the planet, and its rare transits, have provided scientists with crucial scientific data and June's event will be no exception. In particular, astronomers will use it to test techniques they are developing to study the atmospheres of exoplanets – worlds that orbit other suns – and to spot those that possess life-supporting gases such as oxygen and water vapour.

"This is an incredibly hard thing to do," said Dr Suzanne Aigrain of Oxford University. "The light from a star is about a billion times stronger than the light reflected by a planet. So just spotting one in orbit round a star that is many light years away is a considerable achievement. Using that light to analyse a planet's atmosphere is even harder. Nevertheless, the transit of Venus should help us do that."

In the past few years, the study of exoplanets has gone through a revolution with the launch of spacecraft such as the US observatory Kepler. Its telescopes spot tiny falls in light emissions of stars that occur when a planet passes in front of it. Just as the transit of Venus causes a slight dimming of the Sun's light, so an exoplanet reveals its presence when it transits a distant star.

That drop in light provides key data about an exoplanet's size and orbit. However, scientists want to build orbiting observatories that will study tiny changes in the light of a star as it passes through the atmosphere of an orbiting exoplanet. These changes will allow them to assess the composition of that exoplanet's atmosphere and make estimates of surface conditions. Does it have a thick crushing atmosphere or does it possess only thin levels of gas and is therefore unlikely to support life? Does it contain oxygen or does it have poisonous gases? And that is where the transit of Venus should provide crucial data, says Crisp.

"We are developing techniques that will allow us to determine how different exoplanet atmospheres will produce changes in the light from the stars they orbit.

"However, we won't be sure our techniques are right unless we can test them on a planet for which we have precise knowledge – and that, of course, is where Venus comes in. Thanks to probes like Europe's Venus Express, we have precise knowledge about its atmosphere and surface. By studying Venus as if it was an exoplanet we will know how good are our techniques and how much they need to be refined. A whole network of astronomers will be studying the transit of Venus for this reason."

A transit of Venus occurs when the planet and Earth, whose paths round the Sun tilt at slightly different angles, line up exactly where their orbits cross. This occurs only four times every 243 years, in pairs separated by eight years. Only six transits of Venus are known to have been observed (though claims are made for earlier observations by Persian astronomers) with the last, in 2004, watched by millions who used telescopes to project images of the Sun's disc and the dot of Venus on to cards or electronic monitors. After this year's, the next will be in 2117 and then 2125. When the previous pair occurred, Queen Victoria was on the throne.

The first transit of Venus was predicted by Johannes Kepler who calculated one would occur in 1631. However, this was not visible from Europe. The next one occurred on 4 December 1639 when Jeremiah Horrocks became the first person to watch a transit of Venus when he shone an image of the Sun on to a piece of white card and was rewarded, around 3.15pm, with the sight of the black dot of the planet crawling across the solar disc. From his observations, Horrocks used triangulation techniques to make the best estimate then attempted for the size of Venus and the distance of the Earth from the Sun, though in the latter case he was still out by many millions of miles.

The next pair of transits – 1761 and 1769 – got a lot more attention. Expeditions were sent across the globe, including Captain James Cook's first expedition. He visited Tahiti to observe the transit, from a place that is still known as Point Venus. The aim of these various voyages was to collect as many readings as possible from different parts of the world to calculate the size of the solar system with precision.

The expeditions pushed science and many scientists to the limit, the unluckiest being the French astronomer Guillaume Le Gentil, who set out from Paris in March 1760 but was still at sea on transit day, 6 June 1761. The rolling of his ship prevented him from taking observations. So Le Gentil decided to wait for the next transit in 1769 and built a small observatory in Pondicherry, a French colony in India, where he waited patiently for the next transit on 4 June 1769.

On the day, clouds filled the sky even though it had been clear every morning for the preceding month. Le Gentil saw nothing. On his journey home, he contracted dysentery and was caught in a storm that delayed his return to Paris until October 1771 where he found he had been declared legally dead, his wife had remarried and all his relatives had enthusiastically plundered his estate. He eventually remarried, however, and enjoyed an apparently happy life for another 21 years.

Using the observations from the 1761 and 1769 transits, the distance from the Earth to the Sun was estimated as being 153m km. The correct figure, of 149.59m km, was not achieved until results of the transits of 1874 and 1882 were obtained. Today, a Venus transit is of little direct use to astronomers; it is its usefulness in testing transit techniques that now excites scientists.

They are also intriguing events. In 2004, the entire transit was seen in Britain and watched by millions. This time we will catch only a glimpse, with the best observing conditions occurring in Australia, Japan and south-east Asia. In Britain, it will only be possible to watch the last moments of the transit as the sun rises. After that, there will be a lull of 105 years.